The present invention relates to a vehicular cornering lamp system which can change the illumination angle of a headlamp or other vehicular lamp according to the steering angle of the vehicle.
A conventional vehicular cornering lamp system is disclosed in Japanese Patent Publication No. Sho. 64-67439. In the conventional cornering lamp system, a control signal is generated having a pulse width corresponding to the steering angle of the vehicle, a difference between the actual illumination direction of the headlights of the vehicle and the target illumination direction based on the steering angle of the vehicle is detected in accordance with the control signal, and a motor (two-terminal motor) for changing the illumination direction is PWM (pulse-width modulation) controlled in accordance with the difference.
However, in the conventional cornering lamp system, due to the fact that the voltage applied to the two terminals of the motor is in the form of pulses, spark noise is generated when the motor is turned on and off, which has an adverse effect on the transmission and reception properties of wireless communications equipment such as a car radio or the like. Also, the sparks generated when the motor is turned on and off increases the rate of wear of the brushes and commutator of the motor, which reduces the service life of the motor.
Another conventional cornering lamp system includes a motor which changes the illumination angle of the lamp, an illumination angle sensor which outputs a voltage corresponding to the illumination angle of the lamp, a steering angle sensor which outputs a voltage corresponding to the steering angle, and a control circuit which controls the rotation of the motor so that the difference between the output voltages of the illumination angle sensor and steering angle sensor is zero.
However, in the cornering lamp system of the above type, if the steering sensor malfunctions, then the illumination angle of the lamp becomes uncontrollable, which may result in dazzling the driver of an oncoming vehicle or cause other dangerous problems.
An example of a conventional analog control structure for a cornering lamp system is shown in FIG. 8. In this structure, the power voltage of the vehicle's electrical system is applied to one terminal of a motor 1, while the voltage applied to the other terminal of the motor 1 is controlled. That is, a voltage value corresponding to the actual rotation angle is applied from a potentiometer 3 to the inverting input terminal of an operational amplifier 4, a voltage value corresponding to the target rotation angle is applied from a setting device 5 to the noninverting input terminal of the operational amplifier 4, and transistors T1 and T2 are driven on and off in accordance with the output of the operational amplifier 4, thereby controlling the voltage applied to the other terminal of the motor 1.
However, in the above analog control structure, at most only one-half of the power voltage can be applied between the two terminals of the motor 1. For this reason, the power voltage of the vehicle's battery (nominal battery voltage: +12 V) is too low to obtain the torque necessary to drive the motor properly.